As the demand grows for higher throughput, i.e., higher bit rate, and more efficient transmission of packet data over wireless networks, the 3rd Generation Partnership Project (3GPP) has extended its specifications with the High Speed Downlink Packet Access (HSDPA). In HSDPA, a new shared downlink transport channel, called High-Speed Downlink Shared Channel (HS-DSCH), is introduced. This channel is dynamically shared among the packet data users, primarily in the time domain. The application of shared channel makes the use of available radio resources more efficient. The HSDPA also supports new features that rely on the rapid adaptation of transmission parameters to instantaneous radio conditions, also referred to as the Adaptive Modulation and Coding (AMC). The main features of the HSDPA include fast link adaptation, fast Hybrid-ARQ (HARQ—Automatic Repeat Request), and fast channel-dependent scheduling.
Prior to the advent of the HSDPA, transport channels were terminated at the radio network controller (RNC) in wireless networks such as Release '99 networks. This meant that functionalities such as retransmission of packet data, to serve ARQ for example, was located in the RNC. With the advent of the HSDPA, wireless networks such as UMTS (Universal Mobile Telephone System) Terrestrial Radio Access Network (UTRAN), the control of radio frame scheduling is moved from the RNC to the Node-B, i.e., to the base station. In this manner, transmissions and retransmissions of packet data can be directly controlled by the Node-B, which leads to faster retransmissions. This in turn leads to shorter delays and better throughput.
FIG. 1 illustrates an example UTRAN system 100. The system 100 includes an RNC 110 communicating with the core network (not illustrated) over an Iu interface. The system also includes multiple radio base stations also known as Node-Bs 120 connected to the RNC 110 over an Iub interface. The user equipments (UE) 130, typically mobile terminals, communicate with one or more Node-Bs 120 over a Uu interface (radio link). The Iub interface between the Node-B 120 and the RNC 110 has a flow control (FC) mechanism to ensure that the buffers in the Node-B 120 are used properly and to prevent data loss due to buffer overflow.
In UTRAN, fixed capacity (e.g. 64 kbps) can be reserved for traditional Dedicated Channel (DCH) traffic in the access network. However, for HSDPA, per flow bandwidth reservation is not efficient since the Uu interface throughput is much higher and fluctuates more. If the bandwidth reservation is not used, then congestion can occur both in the Iub transport network (TN) between the RNC and the Node-B and also in the Uu interface between the Node-B and the UE. In the current architecture, TCP cannot efficiently resolve a congestion situation in the access network, because lower layer retransmissions hide the congestion situations from the TCP. Thus, a flow control function is introduced to control the data transfer between the RNC and Node-B in HSDPA.
Originally, the flow control (FC) was designed to take only the transmission capabilities of the Uu interface into account and to limit the latency of layer 2 signalling. However, the increased Uu interface capacity did not always coincide with similarly increased Iub TN capacity in practice. The cost of Iub transport links is still high and is not expected to decrease dramatically. It is a common scenario that the throughput is limited by the capacity available on the Iub TN links and not by the capacity of the Uu interface. On these high cost TN links, it is important to maintain high efficiency.
The protocol layers and the nodes involved in the HSDPA are illustrated in FIG. 2. The HSDPA FC is located in the Node-B. The task of the FC is to regulate the transfer of Medium Access Control-d Protocol Data Units (MAC-d PDUs) from the RNC to the Node-B. More precisely, the FC regulates the transfer of data from the Radio Link Control (RLC) buffer in RNC to the MAC-hs buffer in Node-B.
There are at least two types of bottlenecks—the Iub TN and the air interface (Uu) bottlenecks. Typically, the Iub TN bottleneck is a single link between RNC and Node-B, where all flows of the same Node-B share the same TN bottleneck buffer and TN capacity. These flows can utilize the remaining TN capacity from high priority traffic (e.g. DCH). Each flow belonging to the same cell shares Uu resources, but each flow has a dedicated priority queue (PQ) in the Node-B in the form of MAC-hs buffers. A Node-B can include one or more cells. The FC is responsible for efficient use of these changing TN and Uu bottlenecks. It is desirable to maintain high end-user throughput while also maintaining low end-to-end delay for delay sensitive applications such as gaming. The delay target for MAC-d PDUs is typically smaller than 100 ms.
The HSDPA FC is used to avoid or limit Iub TN and Uu congestions. In the Node-B, the flow control function calculates the bit rate of the HSDPA flow and the calculated bit rate is sent to the RNC using the standardized CAPACITY ALLOCATION (CA) control frame. In the RNC, the HSDPA is shaped with this bit rate. Practically, if there is some congestion in the Iub TN or in the Uu part, the flow control function reduces the bit rate of HSDPA flow to resolve the congestion.